I have 3 large cats and I use only 2 boxes downstairs,but they are Rubbermaid storage-boxes,so a bit larger than regular litter-boxes.
I know S-C-M is right,but I could not imagine having 4 boxes in the house and my cats are doing fine with 2.
I scoop them a couple of times daily.
If I was the OP,I would keep the 2 boxes,probably in the same place,but they could get used to a different spot.